linux-kernel - Re: [PATCH v2] ARM: dts: sun8i-a83t: Add thermal trip points/cooling maps

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Date:   Mon, 24 Feb 2020 18:56:18 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Ondřej Jirman <megous@...ous.com>,
        linux-sunxi@...glegroups.com, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:ARM/Allwinner sunXi SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: dts: sun8i-a83t: Add thermal trip points/cooling
 maps

On 24/02/2020 18:39, Ondřej Jirman wrote:
> On Mon, Feb 24, 2020 at 06:23:28PM +0100, megous hlavni wrote:
>> Hi, 
>>
>> On Mon, Feb 24, 2020 at 06:06:20PM +0100, Daniel Lezcano wrote:
>>> On 24/02/2020 17:54, Ondrej Jirman wrote:
>>>> This enables passive cooling by down-regulating CPU voltage
>>>>  			clocks = <&ccu CLK_C1CPUX>;
>>>> @@ -1188,12 +1188,60 @@ cpu0_thermal: cpu0-thermal {
>>>>  			polling-delay-passive = <0>;
>>>>  			polling-delay = <0>;
>>>>  			thermal-sensors = <&ths 0>;
>>>> +
>>>> +			trips {
>>>> +				': cpu-hot {
>>>> +					temperature = <80000>;
>>>> +					hysteresis = <2000>;
>>>> +					type = "passive";
>>>> +				};
>>>> +
>>>> +				cpu0_very_hot: cpu-very-hot {
>>>> +					temperature = <100000>;
>>>> +					hysteresis = <0>;
>>>> +					type = "critical";
>>>> +				};
>>>> +			};
>>>> +
>>>> +			cooling-maps {
>>>> +				cpu-hot-limit {
>>>> +					trip = <&cpu0_hot>;
>>>> +					cooling-device = <&cpu0 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
>>>> +							 <&cpu1 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
>>>> +							 <&cpu2 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>,
>>>> +							 <&cpu3 THERMAL_NO_LIMIT THERMAL_NO_LIMIT>;
>>>> +				};
>>>> +			};
>>>>  		};
>>>>  
>>>>  		cpu1_thermal: cpu1-thermal {
>>>>  			polling-delay-passive = <0>;
>>>
>>> No polling to mitigate?
>>
>> Polling to mitigate what?
>>
>> The driver is using interrupts whenever new reading is available, and
>> notifies tz of the change. I don't have a reason to believe any new
>> values are available from thermal sensor outside of the interrupt
>> period.
> 
> To be more clear, new temperatures are available from the thermal sensor driver
> at the rate of 4 per second, which should be enough to do quick adjustments to
> the thermal zone/cooling device even for quick temperature rises.
> 
> https://elixir.bootlin.com/linux/v5.6-rc3/source/drivers/thermal/sun8i_thermal.c#L442
> 
> There's no slow/fast period depending on whether the cooling is active.
> It's always fast and no polling of the thermal sensor is needed.

Thanks for the clarification. All sensors have their specificity.

Does the sensor allow to create a threshold temperature where an
interrupt fires when crossing the boundary? That would be interesting
for performance and energy saving to disable the interrupts until
'cpu0_hot' is reached, no?

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